Acquisitions and Discontinued Operations - Combined Assets and Liabilities of Discontinued Operations (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	$ 653		$ 617		$ 584	$ 693
Held-to-maturity securities	5,015		4,756
Portfolio loans at fair value	191		147
Loans, net of unearned income	57,381		54,457
Less: Allowance for loan and lease losses	794		848
Net loans	56,587		53,609
Accrued income and other assets	2,952	[1]	3,015	[1]
Total assets	93,821		92,934
Trust accrued expense and other liabilities at fair value	1,621		1,557
Total liabilities	83,279		82,614
Segment, Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and due from banks	19		20
Held-to-maturity securities	1
Loans, net of unearned income	2,104		2,390
Less: Allowance for loan and lease losses	29		39
Net loans	2,266		4,458
Accrued income and other assets	38		45
Total assets	2,324		4,572
Trust securities at fair value			1,834
Total liabilities	3		1,854
Segment, Discontinued Operations [Member] | Seller Note [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Seller note			29
Segment, Discontinued Operations [Member] | Trust [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans at fair value			1,960
Trust accrued income and other assets at fair value			20
Trust accrued expense and other liabilities at fair value			20
Accrued expense and other liabilities	3
Segment, Discontinued Operations [Member] | Portfolio [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Portfolio loans at fair value	$ 191		$ 147

[1] The assets of the VIEs can only be used by the particular VIE, and there is no recourse to Key with respect to the liabilities of the consolidated LIHTC VIEs.